The present invention relates to a highly sensitive magnetic field detecting device using a DC Superconducting quantum interference device (i.e., DC SQUID, which can be used for medical or underground resource searching applications.
FIG. 4 is a block diagram showing the structure of a highly sensitive magnetic field detecting device of the prior art. A reference signal outputted from an oscillator 5 is adjusted by an amplitude adjustor 9 and is converted into an electric current by a first voltage-current converter 6 and is applied as a modulating signal to a DC SQUID 1. When a suitable bias current is fed to the DC SQUID 1 from a variable current source 8, a magnetic flux interlinking the DC SQUID 1 is modulated by the modulating signal and is detected as a modulated signal. This modulated signal is amplified by an amplifier 2 and is detected by a phase detector 3 using the reference signal coming from the oscillator 5. A detected signal corresponding to the output of that phase detector 3 is outputted from an integration circuit 4 and is converted into an electric current by a second voltage-current converter 7. The electric current is applied as such a feedback signal to the DC SQUID 1 so as to compensate a change in the magnetic flux interlinking the DC SQUID 1.
These circuits for driving the DC SQUID 1 are called the "F.L.L. (Flux Locked Loop) circuit device". The magnetic field can be measured by reading out the feedback signal or detected signal. The measurement of a fine magnetic field is performed under a magnetically shielded special circumstance so as to eliminate only disturbing magnetic field. In the prior art, the adjustments of the variable current source 8 and the amplitude adjustor 9 for the measurements described above are accomplished under the circumstance having the disturbing magnetic field or by using a magnetic element such as a magnet to apply noise to the DC SQUID 1 while observing the amplitude of the output coming from the amplifier 2 while being synchronized with the reference signal and the output of the phase detector 3.
Thus, according to the highly sensitive magnetic field detecting device of the prior art, the adjustments of the variable current source 8 and the amplitude adjustor 9 have to be accomplished under a circumstance different from that for the measurements. As a result, measurements under a different circumstance may deviate from those in an optimum state even with an optimum adjustment. If, on the other hand, the DC SQUID 1 used is exemplified by a plane type gradiometer which offsets the influence of the under circumstances other than the special one that is shielded magnetically, there arises a problem in that the signal hardly changes even with disturbances so that the optimum adjustment cannot be achieved.